Question 114486
Make {{{u}}} as the subject in  

{{{1/u+ 1/v = 1/f}}}.........move {{{1/v}}} to the right



 {{{1/u = 1/f - 1/v}}}.........multiply both sides by {{{u}}}

{{{(1/u)u = (1/f - 1/v)u}}}.........

{{{1 = (1/f - 1/v)u}}}.........

{{{1 = (1/f - 1/v)u}}}.........divide both sides by {{{1/f - 1/v}}}

{{{1/(1/f - 1/v) = (1/f - 1/v)u/(1/f - 1/v)}}}.........

{{{1/(1/f - 1/v) = u}}}

or 

{{{u = 1/(1/f - 1/v)}}}

{{{u = (1/1)/((v-f)/fv)}}}

{{{u = fv/(v-f)}}}